This invention relates generally to the ventilation of dynamoelectric machines, and more particularly to an improved method and apparatus for locking the wedges used in such machines having a radially ventilated rotor.
Large turbine generators are usually of the inner cooled construction in which a coolant gas, usually hydrogen, is circulated through ducts in the stator and rotor slots in direct thermal relation with the current-carrying conductors inside the ground insulation. A machine of this type having an improved ventilation system is disclosed and claimed in U.S. Pat. No. 3,110,827, issued Nov. 12, 1963, to R. A. Baudry, assigned to the assignee of the present invention, and incorporated herein by reference.
As described in that patent, more effective cooling of a large turbine generator is obtained by dividing the air gap transversely into a plurality of annular zones by baffle members disposed in the air gap and mounted on the rotor and the stator. Alternate zones are connected to the high pressure or discharge side of a blower mounted on the rotor shaft and the remaining zones are connected to the low pressure or entrance side of the blower. Radial ducts or passages in the rotor in each of them, permit the coolant gas to flow from the hydrogen source to the rotor winding. Thus, the blower pressure is used to force the gas through the rotor ducts in a plurality of short axial paths so that adequate gas flow is obtained and very effective cooling results.
As is well-known, it becomes increasingly difficult to circulate adequate quantities of cooling gas through the rotor ducts from one end of the rotor to the other or to the center as the length of a machine of this type is increased. This is primarily due to the necessarily small cross-sectional area of the ducts. As disclosed in the above-described patent, as well as in an improvement thereto disclosed in U.S. Pat. No. 3,265,912, which was issued to R. A. Baudry, assigned to the assignee of the present invention and incorporated herein by reference, the path of the cooling gas through the rotor ducts is divided into a plurality of relatively short longitudinal paths, and the pressure of the blower is utilized to cause the gas to flow through the short path so that an adequate flow of gas is readily obtained. For this purpose, the air gap is divided transversely into a plurality of annular zones by means of annular baffle members placed in the air gap and extending around the bore of the stator to form the annular zones.
Disposed in the bore of the stator, the rotor is separated from the stator by an annular air gap, and is supported by bearings mounted in the ends of a housing with gland seals being provided to prevent leakage of gas from the housing along the shaft. The rotor is also provided with longitudinal slots in its periphery for the reception of a field winding, the conductors of which extend longitudinally of the rotor and have circumferentially extending end turn portions which are supported against centrifugal forces by retaining rings of usual construction. The windings are insulated from the rotor core by insulating slot cells, with a suitable insulating member being placed at the top of the windings in each slot, and the slot being closed by wedges. A single conductor or damper bar may also be suitably placed between the insulating slot cell and the wedge as part of an amortisseur winding or starter winding.
In order to adequately cool the rotor so as to maximize its efficiency, the rotor is often provided with radial gas passages in each of the zones of the air gap. Thus, radial holes may be drilled through the wedges, damper bar, insulating slot cell, and through the conductors in each slot of the rotor to provide radial passages from the air gap communicating with the longitudinal gas ducts of the rotor winding. Several of these gas passages may be provided in each slot of the rotor in each of the zones of the air gap so that gas may flow from one zone of the air gap of an adjoining zone through the radial passages and longitudinal ducts of the rotor conductors. This flow of gas is effected by connecting alternate zones of the air gap to the high pressure side of the blower and connecting the remaining zones to the low pressure side of the blower, thereby maintaining a pressure differential between adjacent zones of the air gap to cause the desired gas flow through the rotor ducts.
One major problem that occurs in radially ventilated systems is relative movement of the components within each slot. All of the components are sufficiently constrained in the tangential direction by the walls of the slot. Nevertheless, the slot contents are capable of excessive migration axially due to thermal expansion and centrifugal forces caused during rotation. Since the bottom spacer, insulating slot cell, and conductors are continuous throughout the length of the rotor, their axial migrations relative to one another can be prevented through such conventional measures as brazing copper bosses to the bottom surface of each bottom strap at both ends. However, since conventional wedges must be of a short enough length to facilitate their wedging operation, and further since an axial gap must remain between adjacent wedges to allow for their thermal expansion, a possibility exists for the migration of the wedges toward one end of the rotor or the other with an excessive accumulated gap. This situation could cause the closure of the radial holes, thereby obstructing the flow of the required coolant gas.
At both ends of the rotor, the top spacer and the damper bar are axially held in place against the retaining ring and its liner with a small axial clearance. The radial holes formed in each of the contents of the slot, with the exception of the wedge, are sufficiently elongated to accommodate their limited movement during rotation. The entire slot contents (except the wedge) are adequately supported against centrifugal loading. Because the wedges are subjected to high stresses in order to support the centrifugal loading of the slot contents, too long or too many holes to ensure proper ventilation would reduce the strength of the wedge significantly. It would, therefore, be desirable to provide a method and apparatus for locking the wedge segments of a radially ventilated rotor in order to minimize their axial migration.
Other methods and apparatus which lock the wedge segments in place have been proposed in the past. For example, the wedge segments could be locked by prick-punching the wedge material into the tooth top of the rotor slot or vice versa. Not only is such prick-punching capable of bending or damaging the tooth detrimentally, but also catastrophic failure of the tooth is engendered because of the high stress to which the tooth top is subjected. Application of radial set screws between the wedge and tooth top has also been considered. However, potential loosening and loss of the set screws within the operating generator necessitates their elimination in a suitable locking means.